I. Field of the Invention
This invention relates to an apparatus for scribing a semiconductor wafer with a laser beam.
II. Description of the Prior Art
A semiconductor substrate of a semiconductor device is formed by cutting a disc-like semiconductor wafer along dicing lines. The step of cutting is performed by cutting grooves along the dicing lines in the surface of the semiconductor wafer and breaking off the wafer along the grooves to separate it from the rest of the wafer. "Scribing" refers to grooving the semiconductor along the dicing line. The scribing step has heretofore been performed with a laser beam. A conventional scribing apparatus includes a support base and an XY table assembly on the support base. The XY table assembly comprises a first table (X table) movable along the X-axis of the XY coordinate system and a second table (Y table) movable along the Y-axis. The X and Y tables are driven by a pulse motor and DC motor, respectively. A 90.degree.-rotatable vacuum chuck is disposed on the Y table to fixedly suck the semiconductor wafer on the Y table. A laser beam oscillator is located on a diagonally upper side of the Y table. An optical system is arranged above the Y table to permit a laser beam emitted horizontally from the laser beam oscillator to be directed toward the surface of the semiconductor wafer. The optical system comprises a reflective plate for normally shifting the direction of the laser beam emitted horizontally from the laser beam oscillator by 90.degree. and a lens for causing the laser beam reflected by the reflective plate to be focused onto the surface of the wafer.
The operation of the above-mentioned conventional scribing apparatus will be explained below by referring to FIGS. 1 to 3.
FIG. 1 is a timing chart showing a clock pulse sent to the DC motor for driving the Y table and FIG. 2 is a timing chart showing a clock pulse sent to the pulse motor for driving the X table.
First, the semiconductor wafer is fixedly sucked on the vacuum chuck. Then, the DC motor is started at time T.sub.1 as shown in FIG. 1, to move the Y table negatively in the Y axis by an amount substantially corresponding to the diameter of the disc-like wafer. A laser beam is emitted from the laser beam oscillator and through the optical system it forms a groove (scribing line) in the Y axis direction in the surface of the wafer on the vacuum chuck. During this operation, the X table is stopped as shown in FIG. 2. At time T.sub.2, the DC motor is halted to cause the Y table to stop. At the same time, the pulse motor is started as shown in FIG. 2, causing the X table to be shifted negatively in the X axis by a predetermined distance and stopping the X table at time T.sub.3. At time T.sub.3, the Y table is shifted positively in the Y axis by an amount corresponding to the time interval T.sub.3 to T.sub.4. Such operations are repetitively carried out, completing all the scribing operations in the Y direction and driving the DC motor and pulse motor to permit the X and Y tables return to the original positions.
Then, the vacuum chuck is rotated counterclockwise through an angle of 90.degree. and the above-mentioned operation is repeated, completing all the scribing operations in the X axis direction.
FIG. 3 is a view explaining the scribing operations which are effected in the X and Y directions with respect to the wafer 10. The scribing line 12 is formed in the directions indicated by arrows in FIG. 3.
In the conventional scribing apparatus, the laser beam is continuously directed onto the surface of the wafer during the X and Y direction scribing operations including the position shifting operations. In this way, the surface of the wafer is scribed positively and negatively in the X axis. In the Y direction, the scribing operation is also performed positively and negatively. Thus, no inconvenience is encounted when the surface of a semiconductor wafer consisting of silicon is scribed by the conventional scribing apparatus. However, where the surface of a silicon-on-sapphire (SOS) wafer is scribed by the conventional scribing apparatus, microcracks tend to develop in the wafer when the scribing operation is performed in a certain direction. That is, when the SOS wafer is scribed positively and negatively in the X axis, microcracks are likely to occur in one of the directions. When the SOS wafer is scribed by the apparatus positively and negatively in the Y axis, the same defects are likely to occur in one of the directions. When a scribing line about 100 .mu.m thick, for example, which permits the wafer to be broken along the scribing line, was formed by the conventional apparatus in the surface of the SOS wafer about 400 .mu.m, no microcracks were developed along the scribing line in the negative direction of the Y axis, but microcracks occurred along the scribing line in the positive direction of the Y axis. The same defects were also observed in the X axis direction. Such a phenomenon is believed to occur due to the crystallinity or cleavage of the sapphire substrate. If such microcracks occur in the sapphire wafer, damage is caused to the resultant semiconductor element.